Add actions to maintain preview branch

Borrowed from bssw.io repo

.github/workflows/merge-main-to-preview.yml

@@ -0,0 +1,22 @@
+name: Sync main to preview
+
+# When there's a push on main, we want to reflect it on preview too.
+on:
+  push:
+    branches:
+      - 'main'
+jobs:
+  sync-preview:
+    if: github.repository_owner == 'betterscientificsoftware'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@master
+
+      - name: Merge main -> preview
+        uses: devmasx/[email protected]
+        with:
+          type: now
+          head_to_merge: ${{ github.ref }}
+          from_branch: main
+          target_branch: preview
+          github_token: ${{ secrets.GITHUB_TOKEN }}

.github/workflows/merge-pr-to-preview.yml

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+name: Sync pull request to preview
+on:
+  workflow_dispatch:
+  pull_request_target:
+    types: [opened, synchronize, labeled]
+jobs:
+  sync-pull-request:
+    if: contains(github.event.pull_request.labels.*.name, 'preview')
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@master
+        with:
+          ref: 'preview'  # Not default 'main' branch
+      - name: Merge pr -> preview
+        uses: devmasx/[email protected]
+        with:
+          type: now
+          head_to_merge: ${{ github.event.pull_request.head.sha }}
+          target_branch: preview
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+
+# NOTE: Above, we use the 'if' statement for looking for the 'preview' label
+# and then only run the job if that label is found in the PR.  I tried using
+# the setting {label_name: 'preview'} for the `devmasx/merge-branch` action
+# above but it seemed to ignore it and try to merge the PR branch to 'preview'
+# anyway.

.github/workflows/no-prs-on-preview.yml

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+name: Reject pull requests on preview branch
+
+# The preview branch is specially constructed by our workflows and should never be the direct target of a
+# pull request. So if someone tries, we add a polite comment and close the PR.
+
+on:
+  pull_request_target:
+    types: [opened, reopened]
+    branches: 
+      - preview
+
+jobs:
+  reject-pr:
+    runs-on: ubuntu-latest
+    steps:
+# This is a variant of superbrothers/close-pull-request, which only does pull_request events,
+# not pull_request_target events.  Ideally, the capabilities will some day be merged, since
+# the *only* distinction is a test on the type of event!
+    - uses: SiliconLabs/[email protected]
+      with:
+        comment: "The preview branch does not accept pull requests.  To contribute to the CASS website make your PR against the main branch.  Thanks"
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}